Such a transmission system is e.g. a broadband distribution network known from H. Hessenmuller et al. "Access Network Structures for Interactive Video Services", Part 1, The TV Engineer, 48th. Year, August 1994. A summary therein describes broadband distribution networks, which are also called BK networks, and which have A-cable lines, B-cable lines, C-cable lines and D-cable lines established in the network plane 3 (picture 3). The C-cable lines contain the last active elements of the network plane 3, the C-amplifiers. These are the network operator amplifiers that are closest to the subscribers in the BK network. The D-cable lines, which represent access network sections, start at the C-cable line branches and always end at a transfer point. In an upstream BK amplifier station, all TV and Radio programs are combined into a uniform program offer and distributed to the BK amplifier stations on the user side, which are usually found in local exchanges. Starting from the BK amplifier stations, the TV and Radio programs are distributed to the subscribers (customers) in the downstream direction.
To also create the possibility of interactive services in such BK networks, i.e. the so-called service-on-demand (SoD) services such as video-on-demand (VoD), in addition to the unidirectional services in the downstream direction (TV and Radio signals), a (physical) back-channel must be provided in each BK network for sending signals in the upstream direction. This allows subscribers to send useful data to the SoD exchange and to a server network, e.g. to request information. A back-channel and a control channel are required for an interaction between the service offerer and a subscriber. There, among other things, the back-channel supports the identification of the customer, the establishment and termination of the connection, and the interaction of the customer with the SoD exchange and the server. Each customer has an individual back-channel available for the duration of the service usage. In current BK networks a frequency range of about 5-30 MHz is provided for the back-channels, and a frequency range of about 40-300 MHz (450 MHz, hyperband) for the TV channels. One request to the back-channel is e.g. for the reaction time to remain within justifiable limits until a command entered via a terminal (TV set with a set-top box) becomes effective.
Chapter 3.4 of the cited publication investigates solutions for the back-channel, with the result that access or transmission methods for the back-channel in large networks must be developed in multiple stages, and for the case of central access control the center must be shifted in the direction toward the customer. In that respect picture 12 illustrates a two-step back-channel concept. Starting with the general structure of a BK network, a back-channel transmitter with an access control is installed in a C- amplifier. ATM is cited as the transmission mode. The back-channel data from all the terminals of the respective network segment are combined in a time-division multiplex mode, then cell structured and provided with an individual nodal address. Cell multiplexers are located in nodal points of the network having a branching function in the downstream direction.
In this way the coaxial cable-based BK network is used for the back-channel up to the amplifier station on the user side. To make this possible, the BK network must be "back-channel capable", i.e. all amplifiers in the BK network must be able to pass or regenerate signals which expand in the upstream direction (upstream signals). This takes place in that the upstream signals in the back-channel bypass each individual amplifier with the aid of cross-overs, or they are actively amplified together. Making existing BK networks "back-channel capable" is therefore connected with high costs. Furthermore each cross-over also attenuates the downstream signals.